The present invention relates to an aluminum pigment used in a high quality metallic paint field including high quality metallic paints for automobile bodies and parts, metallic paints for automobile repairing, household metallic paints, industrial metallic paints and the like, high quality metallic printing ink field including gravure printing, offset printing, screen printing and the like, and high quality metallic resin additive field, and manufacturing process thereof.
More specifically, the present invention relates to an aluminum pigment having high brightness and high flip-flop effect, which can provide a high light reflectance never seen before and an extremely high flip-flop effect, that is, an optical anisotropy, to paint films, prints or films formed by conventional methods in the above described applications, and manufacturing process thereof.
Aluminum pigments have been popularly used in the above described fields as a pigment having both a peculiar metallic texture which can not be seen with any other pigment and an excellent hiding power.
In recent years, a fashionable characteristic of automobile body painting has become more and more important, and it has been evaluated with the same or higher sense of value than are the intrinsic functions of automobiles. Especially in a trend of the fashion in automobile body painting in recent years, white color or simple glittering silver metallic tone which was deep-seated before, has lost its popularity, and a demand for a paint film having a strong brightness and a strong optical anisotropy, by which appearance of the painting changes depending on a viewing angle, has arisen.
As for the aluminum pigments aiming at obtaining such a high reflectance, and the manufacturing processes thereof, the following is known.
Japanese Patent No. 2575516 describes an aluminum pigment having both a high hiding power and a high light reflectance. JP-A-49-14358 also describes a process to obtain an aluminum pigment having a high reflectance by polishing surfaces of aluminum powder by means of wet ball-mill method. In addition, JP-B-54-36607 describes a process to form a metallic paint film which is excellent in a sparkling effect. Furthermore, JP-A-8-170034 describes an aluminum pigment providing a paint film having strongly brightening texture and excellent appearance. Still further, JP-A-7-509266 discloses metal-powder-pigment particles which are ground almost spherically.
However, none of the art can satisfy high brightness and high flip-flop effect which are presently required for an aluminum pigment.
The present invention provides an aluminum pigment having a high grade of optical characteristics, particularly, a high brightness and a high flip-flop effect, and a manufacturing process which enables manufacturing of such an aluminum pigment very easily with good reproducibility.
In order to solve the above problems, the inventors studied relationships between basic physical properties or optical characteristics of the aluminum pigment and manufacturing conditions earnestly and fundamentally and, as a result, found that the optical characteristics such as high brightness and high flip-flop effect can be revealed by controlling a mean particle diameter, a ratio of the mean particle diameter to the mean thickness and mean surface roughness of an aluminum pigment within respective specified ranges. Thus, the present invention was completed.
The present invention provides an aluminum pigment, which has the mean particle diameter d50 of 5-35 xcexcm, the ratio of the mean particle diameter d50 (xcexcm) to the mean thickness t (xcexcm) of 30-90, and the mean surface roughness Ra of 20 nm or less. The present invention also provides the above described aluminum pigment which further has a mean height of irregularity in the surface roughness curve Rc of 80 nm or less.
In addition, the present invention further provides process for manufacturing the above described aluminum pigment, comprising grinding aluminum by means of a medium mixing mill or ball mill using grinding balls with a specific gravity of 5 or less in a weight ratio of 2-100 to aluminum in a grinding solvent(s). Still further, the present invention provides the above described manufacturing process wherein the diameter of the grinding balls is 4 mm or less in the above manufacturing process.